Flyback converter and control method thereof

ABSTRACT

A conversion device includes a primary side circuit, a secondary side circuit, a transformer, and a control circuit. The primary side circuit includes a primary side switch and is configured to receive an input voltage. The secondary side circuit outputs an output voltage to a load. The transformer comprises a primary winding and a secondary winding, the primary winding is electrically coupled to the primary side circuit and the secondary winding electrically coupled to the secondary side circuit. The control circuit is configured to control a peak value of the current flowing through the primary side switch, to be limited in a band range.

RELATED APPLICATIONS

This application claims priority to China Application Serial Number201810194212.6, filed Mar. 9, 2018, which is herein incorporated byreference.

BACKGROUND Technical Field

The present disclosure relates to a conversion device, and inparticular, to a flyback converter.

Description of Related Art

Flyback converter has been widely used in low power applications,especially in power supply below 100 W, because of its simple circuitstructure and low cost, etc.

Wherein, the flyback converter with quasi-resonant (QR) operation modeis quite popular because it can achieve valley turning on of the primaryside switch which helps to reduce switching loss. However, the switchingfrequency of the QR flyback increases as the output power decreases,which is disadvantageous to the efficiency of the light load condition.

Therefore, a frequency foldback method has been developed to decreasethe switching frequency at light load condition. With the frequencyfoldback method, the switching frequency decreases with decreasing ofthe load, but the peak value of the current also decreases withdecreasing of the load which limits the decreasing speed of theswitching frequency at light load condition. With the development of thehigh-frequency switching power supply, this problem has become more andmore critical.

SUMMARY

One aspect of the present disclosure is provided a conversion device.The conversion device includes a primary side circuit, a secondary sidecircuit, a transformer, and a control circuit. The primary side circuitincludes a primary side switch. The primary side circuit is configuredto receive an input voltage, and the secondary side circuit isconfigured to output an output voltage to a load. The transformercomprises a primary winding and a secondary winding, and the primarywinding is electrically coupled to the primary side circuit and thesecondary winding is electrically coupled to the secondary side circuit.The control circuit is configured to control a peak value of a currentflowing through the primary side switch to be limited in a band range.

Another aspect of the present disclosure is a control method for aconversion device, includes the following operations: receiving an inputvoltage from a primary side circuit and outputting an output voltage toa load through a secondary side circuit; and controlling a peak value ofa current flowing through a primary side switch to be limited in a bandrange.

It is to be understood that both the foregoing general description andthe following detailed description are by examples, and are intended toprovide further explanation of the disclosure as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure can be more fully understood by reading the followingdetailed description of the embodiments, with reference made to theaccompanying drawings as follows:

FIG. 1 is a diagram illustrating a conversion device according to someembodiments of the present disclosure.

FIG. 2A is a diagram illustrating waveforms of the control signal SS₁and the control signal SS_(R), and the drain-source voltage Vds1 of theprimary side switch of a conversion device according to some embodimentsof the present disclosure.

FIG. 2B is a diagram illustrating a relationship of valley ordinal,output power and peak value of the current flowing through the primaryside switch according to some embodiments of the present disclosure.

FIG. 3 is a diagram illustrating a control circuit according to someembodiments of the present disclosure.

FIG. 4 is a diagram illustrating a blanking time control unit accordingto some embodiments of the present disclosure.

FIG. 5 is a diagram illustrating the relationship between a peak valueof the current flowing through the primary side switch and an outputpower according to some embodiments of the present disclosure.

FIG. 6 is a diagram illustrating a relationship between a peak value ofthe current flowing through the primary side switch and an output poweraccording to FIG. 5 of the present disclosure.

FIG. 7 is a diagram illustrating a relationship between an output powerand a switching frequency according to FIG. 5 of the present disclosure.

FIG. 8 is a diagram illustrating a relationship between a peak value ofthe current flowing through the primary side switch and an output poweraccording to some embodiments of the present disclosure.

FIG. 9 is a diagram illustrating a relationship between a peak value ofthe current flowing through the primary side switch and an output poweraccording to according to FIG. 8 of the present disclosure.

FIG. 10 is a diagram illustrating a relationship between an output powerand a switching frequency according to according to FIG. 8 of thepresent disclosure.

FIG. 11 is a flowchart illustrating a control method according to someembodiments of the present disclosure.

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments of the presentdisclosure, examples of which are described herein and illustrated inthe accompanying drawings. While the disclosure will be described inconjunction with embodiments, it will be understood that they are notintended to limit the disclosure to these embodiments. On the contrary,the disclosure is intended to cover alternatives, modifications andequivalents, which may be included within the spirit and scope of thedisclosure as defined by the appended claims. It is noted that, inaccordance with the standard practice in the industry, the drawings areonly used for understanding and are not drawn to scale. Hence, thedrawings are not meant to limit the actual embodiments of the presentdisclosure. In fact, the dimensions of the various features may bearbitrarily increased or reduced for clarity of discussion. Whereverpossible, the same reference numbers are used in the drawings and thedescription to refer to the same or similar parts for betterunderstanding.

The terms used in this specification and claims, unless otherwisestated, generally have their ordinary meanings in the art, within thecontext of the disclosure, and in the specific context where each termis used. Certain terms that are used to describe the disclosure arediscussed below, or elsewhere in the specification, to provideadditional guidance to the practitioner skilled in the art regarding thedescription of the disclosure.

In the following description and in the claims, the terms “include” and“comprise” are used in an open-ended fashion, and thus should beinterpreted to mean “include, but not limited to.” As used herein, theterm “and/or” includes any and all combinations of one or more of theassociated listed items. In this document, the term “coupled” may alsobe termed “electrically coupled,” and the term “connected” may be termed“electrically connected.” “Coupled” and “connected” may also be used toindicate that two or more elements cooperate or interact with eachother. It will be understood that, although the terms “first,” “second,”etc., may be used herein to describe various elements, these elementsshould not be limited by these terms. These terms are used todistinguish one element from another. For example, a first element couldbe termed a second element, and, similarly, a second element could betermed a first element, without departing from the scope of theembodiments.

With reference to FIG. 1, a diagram illustrates a conversion device 100according to some embodiments of the present disclosure. Wherein, theconversion device 100 is configured to convert the input voltage V_(in)received from the input voltage source to output voltage V_(o) andoutput the output voltage V_(o) to load R_(L). In some embodiments,conversion device 100 may be a flyback converter, specifically, in someother embodiments, the conversion device 100 may be an active clampflyback converter, but the present disclosure is not limited thereto.

As illustrated in FIG. 1, the conversion device 100 includes atransformer 110, a primary side circuit 130, a secondary side circuit150 and a control circuit 170. The transformer 110 includes a primarywinding and a secondary winding. The primary winding is electricallycoupled to the primary side circuit 130, and the secondary winding iselectrically coupled to the secondary side circuit 150. The primary sidecircuit 130 includes an input power source, a clamp circuit 132 and aprimary side switch S₁. The secondary side circuit 150 includes asecondary side switch S_(R), a capacitor C_(o) and a load R_(L). Theclamp circuit 132 further includes a clamp resistor R_(clamp), a clampcapacitor C_(clamp), and a diode D_(clamp).

As shown in FIG. 1, a first side of the primary winding is electricallycoupled to a first side of the input power source; a second side of theprimary winding is electrically coupled to the ground terminal and asecond side of the input power source through the primary side switchS₁. The first side of the secondary winding is electrically coupled to afirst side of the capacitor C_(o) through the secondary side switchS_(R), the second side of the secondary winding is electrically coupledto the second terminal of the capacitor C_(o). The transformer 110 isconfigured to transmit the received electrical energy from the primarywinding to the secondary winding. In conjunction with the cooperativeoperation of circuit elements such as the primary side switch S₁ and thesecondary side switch S_(R), the input voltage V_(in) is converted tothe output voltage V_(o) to output to the load R_(L). Wherein, theconversion device 100 illustrated in FIG. 1 is only one of many possibleimplementations of the present disclosure, but the present disclosure isnot limited thereto.

The output power of the conversion device 100 with quasi-resonantcontrol or frequency foldback control is calculated by the followingformula:P _(o)=½L _(m) I _(pk) ² f _(s)  (1)Wherein P_(o) is the output power; L_(m) represents the inductance valueof the transformer 110; I_(pk) is the peak value of the current flowingthrough the primary side switch S₁; f_(s) is the switching frequency ofthe primary side switch S₁.

Referring to the formula above, if the peak value of the current I_(pk)is fixed, the switching frequency f_(s) decreases proportionally to theoutput power P_(o). When the peak value of the current I_(pk) maintainsa maximum value which is limited by the transformer 110, the controlmethod may achieve the best frequency decreasing at light loadcondition. Wherein, the fixed peak value of the current represents thatthe on time t_(on) of the primary side circuit 130 is fixed, so thiscontrol method is also called a fixed on-time control method.

However, if the peak value of the current I_(pk) is fixed, the switchingfrequency f_(s) is continuously adjustable with the output power P_(o),so it may not be guaranteed that the primary side switch S1 can beturned on at the valley for any output power condition, which willresult in inefficiency in some cases when the primary side switch is notturned on at the valley.

In order to turn on the primary side switch at the valley at any outputpower P_(o), the peak value of the current I_(pk) needs to be changedfrom fixing at a certain value to fixing within a band range. Wherein,the band range has an upper limit value and a lower limit value. Whenthe peak value of the current I_(pk) is fixed within a band range, theconduction time of the primary side switch S₁ is correspondingly limitedwithin a range and is approximately fixed. This control method is calleda quasi-fixed on-time control method.

The conversion device 100 satisfies the following formula:

$\begin{matrix}{I_{PK} = {\frac{V_{in}}{L_{m}}t_{on}}} & (2) \\{I_{PK} = \frac{n\; V_{o}}{L_{m}t_{off}}} & (3)\end{matrix}$wherein I_(pk) is the peak value of the current flowing through theprimary side switch S₁; L_(m) is the inductance value of the transformer110; V_(in) represents the input voltage; t_(on) is the on time of theprimary side switch S₁; n is the ratio of turns of the primary windingand secondary winding of the transformer 110; V_(o) is the outputvoltage; t_(off) is the turn on time of the secondary side switch S_(R).

After the current in the secondary circuit 150 drops to zero, thedrain-source voltage V_(DS) of the primary side switch S₁ beginoscillating. The oscillation is caused by the inductor of thetransformer 110 and the parasitic capacitors. The period of theoscillation is:T _(r_ECQ)=2π√{square root over (L_(m) C _(EQ))}  (4)wherein T_(r_ECQ) is the resonance period. Lm is the inductance value ofthe transformer 110. C_(EQ) is the parasitic capacitors value of theprimary side switch S₁ and the transformer 110.

The method of frequency foldback is based on the quasi-resonant workingstate, and with the decrease of output power, a blanking time t_(d) isinserted, so that the conversion device 100 works in discontinuous mode.If the inserted blanking time t_(d) satisfies the following formula, theprimary side switch S₁ can be turned on at the valley.

$\begin{matrix}{t_{d} = {\left( {m + \frac{1}{2}} \right)T_{r\_{CEQ}}}} & (5)\end{matrix}$wherein t_(d) is the blanking time. m is a non-negative integer, whichis the valley ordinal.

The switching frequency f_(s) of the primary side switch S₁ satisfiesthe following formula:

$\begin{matrix}{f_{s} = \frac{1}{t_{on} + t_{off} + t_{d}}} & (6)\end{matrix}$wherein t_(on) is the conduction time of the primary side switch S₁.t_(off) is the conduction time of the secondary side switch S_(R). t_(d)is the blanking time.

Referring to FIG. 2A, t2 to t3 is the conduction time t_(on) of theprimary side switch S₁, t3 to t4 is the conduction time t_(off) of thesecondary side switch S_(R), t4 to t5 is blanking time t_(d). From t4 tot5, the control signal SS₁ and the control signal SS_(R) are controlledat low level, both the primary side switch S₁ and the secondary sideswitch S_(R) are off, and the drain-source voltage V_(DS) of the primaryside switch S₁ starts to oscillate. The blanking time t_(d) may becontrolled to be ended at the valley of the resonance, and the primaryside switch S₁ can be turned on at the valley of the resonance at thesame time.

From the above formula (2), (3), (5), (6), the following formula may beobtained:

$\begin{matrix}{{P_{o}\left( {m,I_{pk}} \right)} = {\frac{1}{2}L_{m}I_{pk}^{2}\frac{1}{\frac{L_{m}I_{pk}}{v_{in}} + \frac{L_{m}I_{pk}}{n\; V_{o}} + {\left( {m,\frac{1}{2}} \right)*2\;\pi\sqrt{L_{m}C_{EQ}}}}}} & (7)\end{matrix}$

From the above formula (7), for any one output power P_(o), severalgroups (m, I_(pk)) corresponding to the output power P_(o) may be found,and the primary side switch S₁ can be turned on at the valley with anyone of the groups (m, I_(pk)).

Further, from the above formula, for any one output power P_(o), thecorresponding different group (m, I_(pk)) has the followingrelationship: the smaller the m, the smaller the I_(pk).

As illustrated in FIG. 2B, for different output power P_(o1), P_(o2),P_(o3), . . . , P_(oN), the corresponding (m₁, I_(pk1)) (m₂, I_(pk2))(m₃, I_(pk3)) . . . , (M_(N), I_(pkN)) exists, which makes the peakvalue of current I_(pk1), I_(pk2), I_(pk3) . . . I_(pkN) relativelyclose. Therefore, a band range can be set to include I_(pk1), I_(pk2),I_(pk3) . . . I_(pkN). Due to the peak value of the current I_(pk) islimited to the band range, the peak value of the current I_(pk) flowingthrough the primary side circuit 130 does not decrease with thedecreasing of the output power P_(o), so that a better frequencydecreasing at light load condition can be achieved, and the primary sideswitch can be turned on at the valley.

The valley ordinal m mentioning above does not have to be increased ordecreased in consecutive integers, but the adjacent valley ordinal mmust satisfy the following formula:

$\begin{matrix}\left\{ \begin{matrix}{{P_{o}\left( {m_{1},A} \right)} \leq {P_{o}\left( {m_{2},B} \right)}} \\{m_{1} < m_{2}}\end{matrix} \right. & (7)\end{matrix}$wherein m₁ and m₂ are adjacent integers. A is a lower limit value of theband range, and B is an upper limit value of the band range.

In combination with the formula (1), (2), (3), (6) above, the followingformula (8) may be obtained:

$\begin{matrix}{{P_{o}\left( {m,f_{s}} \right)} = {\frac{1}{2}{L_{m}\left( \frac{\frac{1}{f_{s}} - {\left( {m + \frac{1}{2}} \right)T_{r\_{CEQ}}}}{\frac{L_{m}}{v_{in}} + \frac{L_{m}}{{nV}_{o}}} \right)}^{2}{fs}}} & (8)\end{matrix}$

Since the peak value of the current I_(pk) is proportional to a feedbackvoltage V_(FB), the peak value of current I_(pk) may be controlled in aband range as shown in FIG. 2B by controlling the feedback voltageV_(FB) in a voltage threshold range. In some embodiments, the controlcircuit 170 obtains the feedback voltage V_(FB) from output voltageV_(o) of the secondary side circuit 150. The voltage threshold range hasa voltage upper limit value V_(FB_H) and voltage lower limit valueV_(FB_L).

Wherein, the control circuit sets a current setting value according tothe feedback voltage V_(FB). When the current Ip of the primary sidecircuit 130 reaches the current setting value, the control circuit 170outputs a turn off signal to the primary side switch S₁. When thefeedback voltage V_(FB) exceeds the voltage threshold range, the controlcircuit 170 adjusts the blanking time of the primary side switch S₁, andan ending signal S_END is outputted according to the blanking time.Further, the control circuit 170 outputs the turn on signal to turn onthe primary side switch S₁ according to the ending signal S_END and avalley conduction signal S_DET.

In some embodiments, when the feedback voltage V_(FB) is larger than thevoltage upper limit value V_(FB_H) of the voltage threshold range, thecontrol circuit 170 increases the switching frequency f_(s) of theprimary side switch S₁. When the feedback voltage V_(FB) is smaller thanthe voltage lower limit value V_(FB_L) of the voltage threshold range,the control circuit 170 decreases the switching frequency f_(s) of theprimary side switch S₁.

In some embodiments, when the feedback voltage V_(FB) is larger than thevoltage upper limit value V_(FB_H) of the voltage threshold range, thecontrol circuit 170 decreases the blanking time. When the feedbackvoltage V_(FB) is smaller than the lower limit value V_(FB_L) of thevoltage threshold range, the control circuit 170 increases the blankingtime.

Referring to FIG. 3, the control circuit 170 includes a turn off controlunit 172, a blanking time control unit 174, a detection unit 176 and aconduction control unit 178. The control circuit 170 is for illustrativepurposes only, but the present disclosure is not limited thereto.

Referring to FIG. 1 and FIG. 3, the turn off control unit 172 receivesthe current I_(p) of the primary side circuit 130. When the currentI_(p) of the primary side circuit 130 is equal to the current settingvalue corresponding to the feedback voltage value V_(FB), the turn offcontrol unit 172 outputs the turn off signal S_OFF to turn off theprimary side switch S₁.

The blanking time control unit 174 receives the feedback voltage V_(FB),and adjusts the blanking time according to the feedback voltage V_(FB)and the voltage threshold range, and outputs the ending signal S_ENDaccording to the blanking time.

Further, the detection unit 176 is configured to detect the valleymoment of the drain-source voltage V_(DS) of the primary side switch S₁,and transmits a valley conduction signal S_DET to the conduction controlunit 178 at a valley moment.

The conduction control unit 178 is configured to output a turn on signalS_ON to the primary side switch S₁ according to the ending signal S_ENDand the valley conduction signal S_DET. In some embodiments, theconduction control unit 178 is further configured to turn on the primaryside switch S₁ at the moment of the first valley after receiving theending signal S_END.

Referring to FIG. 4, a diagram illustrates a blanking time control unit174 as shown in FIG. 3 according to some embodiments of the presentdisclosure. The blanking time control unit 174 includes a comparisonunit 174 a, an adjusting unit 174 b and a timing unit 174 c. Theadjusting unit 174 b further includes a first adjusting unit 174 b 1 anda second adjusting unit 174 b 2. The blanking time control unit 174illustrated in FIG. 4 is for illustrative purposes only, and the presentdisclosure is not limited thereto.

Further, the comparison unit 174 a is configured to determine whetherthe feedback voltage V_(FB) exceeds the voltage threshold range or not.When the feedback voltage V_(FB) is beyond the voltage threshold range,a comparison signal S_COM is transmitted to the adjusting unit 174 b bythe comparison unit 174 a. The adjusting unit 174 b is configured toreceive the comparison signal S_COM, adjust the blanking time accordingto the comparison signal S_COM, and transmit the blanking control signalS_T to the timing unit 174 c. The timing unit 174 c is configured tocontrol the blanking time according to the blanking control signal S_Tand output the ending signal S_END.

In some embodiments, when the feedback voltage V_(FB) is larger than thevoltage upper limit value V_(FB_H) of the voltage threshold range, thefirst adjusting unit 174 b 1 decreases a unit time value Δt at a timeuntil the feedback voltage V_(FB) enters to the voltage threshold range.When the feedback voltage V_(FB) is smaller than the voltage lower limitvalue V_(FB_L) of the voltage threshold range, the second adjusting unit174 b 2 increases a unit time value Δt at a time to adjust the blankingtime until the feedback voltage V_(FB) enters to the voltage thresholdrange.

In some embodiments, the unit time value is equal to the time betweenany two valleys. In other words, when the feedback voltage V_(FB) islarger than the voltage upper limit value V_(FB_H) of the voltagethreshold range, the first adjusting unit 174 b 1 decreases the numberof valley to adjust the blanking time. When the feedback voltage V_(FB)is smaller than the voltage lower limit value V_(FB_L) of the voltagethreshold range, the second adjusting unit 174 b 2 increases the numberof valley to adjust the blanking time.

FIG. 5 is a diagram 500 illustrating the relationship between the peakvalue of the current I_(pk) and an output power P_(o) according to someembodiments of the present disclosure. The curves X0 to XM shown in FIG.5 represent the relationship curve of the peak value of the currentI_(pk) versus output power P_(o) for different values of m. FIG. 7 is arelationship diagram 700 illustrating a relationship between an outputpower P_(o) and a switching frequency f_(s) according to FIG. 5 of thepresent disclosure.

Assume that the current lower limit value I_(PK_L) corresponding to thevoltage lower limit value V_(FB_L) is 2.292 A, and the current upperlimit value I_(PK_H) corresponding to the voltage upper limit valueV_(FB_H) is 2.75 A. For example, as shown in FIG. 5-7, when the outputpower P_(o) starts to decrease from the full load P_(max), at this time,the control circuit 170 sets m=0, which represents that the primary sideswitch S₁ is turned on at the first valley. With the decrease of theoutput power P_(o), the primary side switch S₁ remains to be turned onat the first valley, and the peak value of the current I_(pk) decreasesgradually, and the switching threshold f_(s) increases gradually. Atthis time, the relationship curve of the peak value of the currentI_(pk) versus the output power P_(o) is the curve X0.

When the peak value of the current I_(pk) decreases to be lower than thecurrent lower limit value I_(PK_L) corresponding to the voltage lowerlimit value V_(FB_L), the control circuit 170 sets m=1, so the controlcircuit 170 controls the primary side switch S₁ to be turned on at thesecond valley. At this time, the peak value of the current I_(pk) has astep increase and the switching frequency f_(s) has a step reductioncompared to m=0. Then, with the decrease of the output power P_(o), theprimary side switch S₁ remains to be turned on at the second valley.Meanwhile, the peak value of the current I_(pk) gradually decreases andthe switching frequency f_(s) gradually increases with the decrease ofthe output power P_(o). As shown in FIG. 5, the relationship curve ofthe peak value of the current I_(pk) versus the output power P_(o) isthe curve X1.

When the peak value of the current I_(pk) is decreased to be lower thanthe current lower limit value I_(PK_L) corresponding to the voltagelower limit value V_(FB_L) again, the control circuit 170 sets m=2, sothe control circuit 170 controls the primary side switch S₁ to be turnedon at the third valley. At this time, the relationship curve of the peakvalue of the current I_(pk) versus the output power P_(o) is the curveX2. And so on, the switching frequency f_(s) is gradually adjusted,until the switching frequency f_(s) decreases to a frequency settingvalue. For example, the control circuit 170 sets m=M, at this time, therelationship curve of the peak value of the current I_(pk) versus theoutput power P_(o) is the curve XM.

FIG. 6 is a relationship diagram 600 illustrating a relationship betweena peak value of the current I_(pk) and an output power P_(o) accordingto FIG. 5 of the present disclosure. As illustrated in FIG. 6, theconversion device 100 in the present disclosure may effectively controlsthe peak value of the current I_(pk) to be within a band rangecorresponding to the voltage threshold range.

As illustrated in FIG. 7, the dashed line represents relationshipbetween switching frequency f_(s) and output power P_(o) with the fixedon-time control method. The solid line represents relationship betweenswitching frequency f_(s) and output power P_(o) in the presentdisclosure. Wherein, the switching frequency f_(s) decreases segmentallywith the decrease of the output power P_(o), and fluctuates up and downthe dashed line to form a substantially downward trend.

FIG. 8 is a relationship diagram 800 illustrating a relationship betweena peak value of the current I_(pk) and an output power P_(o) accordingto some embodiments of the present disclosure. The curves X0 to XM shownin FIG. 8 represent the relationship curve of the peak value of thecurrent I_(pk) versus output power P_(o) for different values of m. FIG.10 is a diagram 1000 illustrating a relationship between an output powerP_(o) and the switching frequency f_(s) according to according to FIG. 8of the present disclosure.

Assume that the current lower limit value I_(PK_L) corresponding to thevoltage lower limit value V_(FB_L) is 2.292 A, and the current upperlimit value I_(PK_H) corresponding to the voltage upper limit valueV_(FB_H) is 2.75 A. As shown in FIG. 8-10, when the output power P_(o)increases from the light load condition, for example, m=M, whichrepresents that the primary side switch S₁ is turned on at the M+1valley. With the increase of the output power P_(o), the primary sideswitch S₁ remains to be turned on at the M+1 valley. The peak value ofthe current I_(pk) increases gradually, and the switching frequencyf_(s) decreases gradually. At this time, the relationship curve of thepeak value of the current I_(pk) versus the output power P_(o) is thecurve XM.

When the peak value of the current I_(pk) is increased to be higher thanthe current upper limit value I_(PK_H) corresponding to the voltageupper limit value V_(FB_H), the control circuit 170 sets m=M−1, so thecontrol circuit 170 controls the primary side switch S₁ to be turned onat the M valley. At this time, the peak value of the current I_(pk) hasa step down and the switching frequency f_(s) has a step increasecompared to m=M. With the increase of the output power P_(o), theprimary side switch S₁ remains to be turned on at the M valley, and thepeak value of the current I_(pk) increases gradually, and the switchingfrequency f_(s) decreases gradually. At this time, the relationshipcurve of the peak value of the current I_(pk) versus the output powerP_(o) is the curve XM−1.

When the peak value of the current I_(pk) is increased to be higher thanthe current upper limit value I_(PK_H) corresponding to the voltageupper limit value V_(FB_H), the control circuit 170 sets m=M−2, so theprimary side switch S₁ is turned on at the M−1 valley. At this time, therelationship curve of the peak value of the current I_(pk) versus theoutput power P_(o) is the curve XM−2. And so on, the switching frequencyf_(s) is adjusted gradually, for example, the control circuit 170 setsm=0, at this time, the relationship curve of the peak value of thecurrent I_(pk) versus the output power P_(o) is the curve X0, and theoutput power P_(o) reaches the full load P_(max).

FIG. 9 is a diagram 900 illustrating a relationship between a peak valueof the current I_(pk) and an output power P_(o) according to accordingto FIG. 8 of the present disclosure. As illustrated in FIG. 9, theconversion device 100 in the present disclosure may effectively controlsthe peak value of the current I_(pk) within a band range correspondingto the voltage threshold range.

As illustrated in FIG. 10, the dashed line represents relationshipbetween switching frequency f_(s) and output power P_(o) with the fixedon-time control method. The solid line represents relationship betweenswitching frequency f_(s) and output power P_(o) in the presentdisclosure. The switching frequency f_(s) increases segmentally with theincrease of the output power P_(o), and fluctuates up and down thedashed line to form a substantially upward trend.

Referring to FIG. 7 and FIG. 10, the present disclosure approximatelyrealizes the linear decrease of the switching frequency f_(s) with thedecrease of the output power P_(o), a better frequency decreasing atlight load condition is achieved, and at the same time, the zero voltageconduction of the primary side switch S₁ is guaranteed.

With reference to FIG. 11, a flowchart 1100 illustrates a control methodaccording to some embodiments of the present disclosure. For convenienceand clarity, the following control method 1100 is described incoordinate with the examples shown in FIG. 1, FIG. 3, and FIG. 4, butthe present disclosure is not limited thereto. Anyone who is familiarwith this skill can make various changes and refinements withoutdeparting from the spirit and scope of the present disclosure.

As illustrated in FIG. 11, the control method 1100 includes proceduresS1130, S1150 and S1170. In operation S1130, when the current I_(p) ofthe primary side circuit 130 reaches the current setting value, thecontrol circuit 170 outputs the turn off signal S_OFF to the primaryside switch S₁ to turn off the primary side switch S₁.

In operation S1150, when the feedback voltage V_(FB) is beyond thevoltage threshold range, the control circuit 170 adjusts the blankingtime of the primary side switch S₁, and outputs the ending signal S_ENDaccording to the blanking time. In some embodiments, the voltagethreshold range includes the voltage upper limit value V_(FB_H) and thevoltage lower limit value V_(FB_L). For example, when the feedbackvoltage V_(FB) is larger than the voltage upper limit value V_(FB_H) ofthe voltage threshold range, the control circuit 170 decreases theblanking time. When the feedback voltage V_(FB) is smaller than thevoltage lower limit value V_(FB_L) of the voltage threshold range, thecontrol circuit 170 increases the blanking time. Then, the controlcircuit 170 outputs the ending signal S_END according to the blankingtime.

In some other embodiments, when the feedback voltage V_(FB) is largerthan the voltage upper limit value V_(FB_H) of the voltage thresholdrange, the control circuit 170 increases the switching frequency f_(s)of the primary side switch S₁ to decrease the blanking time. When thefeedback voltage V_(FB) is smaller than the voltage lower limit valueV_(FB_L) of the voltage threshold range, the control circuit 170decreases the switching frequency f_(s) of the primary side switch S₁ toincrease the blanking time. Then, the control circuit 170 outputs theending signal S_END according to the blanking time.

Further, the comparison unit 174 a as illustrated in FIG. 4 determineswhether the feedback voltage V_(FB) is beyond the voltage thresholdrange or not, and when the feedback voltage V_(FB) is beyond the voltagethreshold range, the comparison unit 174 a transmits the comparisonsignal S_COM to the adjusting unit 174 b. The adjusting unit 174 breceives the comparison signal S_COM, adjusts the blanking timeaccording to the comparison signal S_COM, and transmits the blankingcontrol signal S_T to the timing unit 174 c. The timing unit 174 c setsand records the blanking time according to the blanking control signalS_T, and outputs the ending signal S_END.

In operation S1170, the control circuit 170 outputs the turn on signalS_ON to the primary side switch S₁ according to the ending signal S_ENDand the valley conduction signal S_DET to turn on the primary sideswitch S₁. In some embodiments, the conduction control unit 178 asillustrated in FIG. 3 outputs the turn on signal S_ON to the primaryside switch S₁ according to the ending signal S_END and the valleyconduction signal S_DET to turn on the primary side switch S₁. Forexample, the conduction control unit 178 as illustrated in FIG. 3 isfurther configured to turn on the switch S₁ at the first valley occurmoment after receiving the ending signal S_END. For another example, ifthe control circuit 170 controls m=M−1, at this time, the controlcircuit 170 outputs the turn on signal S_ON to the primary side switchS₁ according to the ending signal S_END and the valley conduction signalS_DET, to control the primary side switch S₁ to be turned on at the Mvalley.

In some embodiments, the ending signal S_END is transmitted to the turnon control unit 178 by the blanking time control unit 174. In someembodiments, the detection unit 176 detects the valley moment of thedrain-source voltage V_(DS) of the primary side switch S₁, and transmitsthe valley conduction signal S_DET to the conduction control unit 178 atthe valley moment.

In summary, in the embodiments of the present disclosure, the primaryside switch S₁ is turned off when the current I_(p) of the primary sidecircuit 130 reaches the current setting value corresponding to thefeedback voltage value V_(FB), and the switching frequency f_(s) of theprimary side switch S₁ is controlled by adjusting the blanking time tolimit the peak value of the current I_(pk) within a band range. In theembodiments of the present disclosure, the peak value of the current ofthe primary circuit 130 is not required to decrease with the deceasingof the output power P_(o), and the better frequency decreasing at lightload condition can be achieved, and the primary side switch can be tunedon at valley to guarantee efficiency.

Those skilled in the art can immediately understand how to perform theoperations and functions of the control method 1100 based on theconversion device 100 in the various embodiments described above, andthus a further explanation is omitted herein for the sake of brevity.

While disclosed methods are illustrated and described herein as a seriesof acts or events, it will be appreciated that the illustrated orderingof such acts or events are not to be interpreted in a limiting sense.For example, some acts may occur in different orders and/or concurrentlywith other acts or events apart from those illustrated and/or describedherein. In addition, not all illustrated acts may be required toimplement one or more aspects or embodiments of the description herein.Further, one or more of the acts depicted herein may be carried out inone or more separate acts and/or phases.

Although the disclosure has been described in considerable detail withreference to certain embodiments thereof, it will be understood that theembodiments are not intended to limit the disclosure. It will beapparent to those skilled in the art that various modifications andvariations can be made to the structure of the present disclosurewithout departing from the scope or spirit of the disclosure. In view ofthe foregoing, it is intended that the present disclosure covermodifications and variations of this disclosure provided they fallwithin the scope of the following claims.

What is claimed is:
 1. A conversion device, comprising: a primary sidecircuit, comprising a primary side switch, and configured to receive aninput voltage; a secondary side circuit, output an output voltage to aload; a transformer comprising a primary winding and a secondarywinding, and the primary winding electrically coupled to the primaryside circuit and the secondary winding electrically coupled to thesecondary side circuit; and a control circuit configured to control apeak value of the current flowing through the primary side switch, to belimited in a band range; wherein when the peak value of the current ofthe primary side switch is larger than an upper limit value of the bandrange, the control circuit increases the switching frequency: when thepeak value of the current of the primary side switch is smaller than alower limit value of the band range, the control circuit decreases theswitching frequency.
 2. The conversion device of claim 1, wherein whenthe peak value of the current of the primary side switch reaches acurrent setting value, the control circuit outputs a turn off signal toturn off the primary side switch; wherein when the peak value of thecurrent of the primary side switch is beyond the band range, the controlcircuit adjusts a blanking time of the primary side switch, and outputsan ending signal according to the blanking time; wherein the controlcircuit outputs a turn on signal to the primary side switch according tothe ending signal and a valley conduction signal to turn on the primaryside switch.
 3. The conversion device of claim 1, wherein when the peakvalue of the current of the primary side switch is larger than the upperlimit value of the band range, the control circuit decreases theblanking time; when the peak value of the current of the primary sideswitch is smaller than the lower limit value of the band range, thecontrol circuit increases the blanking time.
 4. The conversion device ofclaim 2, wherein the control circuit further comprises: a turn offcontrol unit, when the peak value of the current of the primary sideswitch reach the current setting value which is determined by a feedbackvoltage, the turn off control unit outputs the turn off signal to theprimary side switch.
 5. The conversion device of claim 2, wherein thecurrent setting value is proportional to a feedback voltage, wherein thecontrol circuit further comprises: an blanking time control unit,configured to adjust the blanking time according to the feedback voltageand a voltage threshold range, and outputs the ending signal accordingto the blanking time; and a conduction control unit, configured tooutputs the turn on signal to the primary side switch according to theending signal and the valley conduction signal.
 6. The conversion deviceof claim 5, wherein the blanking time control unit further comprises: acomparison unit, configured to compare the feedback voltage with thevoltage threshold range, and transmits a comparison signal when thefeedback voltage value is beyond the voltage threshold range; anadjusting unit, configured to receive the comparison signal, and adjustthe blanking time according to the comparison signal, and transmit ablanking control signal; and a timing unit, configured to set and recordthe blanking time according to the blanking control signal, and outputsthe ending signal.
 7. The conversion device of claim 2, wherein thecontrol circuit further comprises: a detection unit, configured todetect a valley moment of a drain-source voltage of the primary sideswitch, and transmit the valley conduction signal to the conductioncontrol unit during the valley moment.
 8. The conversion device of claim7, wherein the conduction control unit is further configured to turn onthe primary side switch at a first valley moment after receiving theending signal.
 9. The conversion device of claim 6, wherein when thefeedback voltage is larger than a voltage upper limit value of thevoltage threshold range, the adjusting unit decreases a unit time valueat a time until the feedback voltage enters to the voltage thresholdrange; when the feedback voltage is smaller than a voltage lower limitvalue of the voltage threshold range, the adjusting unit increases aunit time value at a time until the feedback voltage enters to thevoltage threshold range.
 10. The conversion device of claim 9, whereinthe unit time value is equal to the time between any two valleys.
 11. Acontrol method for a conversion device, comprising: receiving an inputvoltage from a primary side circuit and outputting an output voltage toa load through a secondary side circuit; and controlling a peak value ofthe current flowing through a primary side switch of the primary sidecircuit to be limited in a band range by a control circuit; increasing aswitching frequency of the primary side switch by the control circuitwhen the peak value of the current of the primary side switch is largerthan an upper limit value of the band range; and decreasing a switchingfrequency of the primary side switch by the control circuit when thepeak value of the current of the primary side switch is smaller than alower limit value of the band range.
 12. The control method of claim 11,further comprising: outputting a turn off signal to the primary sideswitch by the control circuit to turn off the primary side switch whenthe peak value of the current of the primary side switch reaches acurrent setting value; adjusting a blanking time of the primary sideswitch when the peak value of the current of the primary side switch isbeyond the band range, and outputting an ending signal according to theblanking time by the control circuit; and outputting a turn on signal tothe primary side switch by the control circuit according to the endingsignal and a valley conduction signal to turn on the primary sideswitch.
 13. The control method of claim 11, further comprising:decreasing the blanking time by the control circuit when the peak valueof the current of the primary side switch is larger than the upper limitvalue of the band range; and increasing the blanking time by the controlcircuit when the peak value of the current of the primary side switch issmaller than the lower limit value of the band range.
 14. The controlmethod of claim 12, further comprising: outputting the turn off signalto the primary side switch by a turn off control unit to turn off theprimary side switch when the peak value of the current of the primaryside switch reaches the current setting value which is determined by afeedback voltage.
 15. The control method of claim 12, wherein thecurrent setting value is proportional to a feedback voltage, and thecontrol method further comprises: adjusting the blanking time by ablanking time control unit according to the feedback voltage and avoltage threshold range, and outputting an ending signal to a conductioncontrol unit according to the blanking time; and outputting the turn onsignal to the primary side switch by the conduction control unitaccording to the ending signal and the valley conduction signal to turnon the primary side switch.
 16. The control method of claim 15, furthercomprising: comparing the feedback voltage with the voltage thresholdrange by a comparison unit, and transmitting a comparison signal whenthe feedback voltage is beyond the voltage threshold range; receivingthe comparison signal by an adjusting unit, adjusting the blanking timeaccording to the comparison signal, and outputting a blanking controlsignal; and setting and recording the blanking time by a timing unitaccording to the blanking control signal, and outputting the endingsignal.
 17. The control method of claim 12, further comprising:detecting a valley moment of a drain-source voltage of the primary sideswitch by a detection unit, and transmitting the valley conductionsignal to the conduction control unit at the valley moment.
 18. Thecontrol method of claim 17, further comprising: conducting the primaryside switch by the conduction control unit at the first valley momentafter receiving the ending signal.
 19. The control method of claim 16,further comprising: decreasing the blanking time by a unit time value ata time by the adjusting unit until the feedback voltage enters to thevoltage threshold range when the feedback voltage value is larger than avoltage upper limit value of the voltage threshold range; and increasingthe blanking time by a unit time value at a time by the adjusting unituntil the feedback voltage enters to the voltage threshold range whenthe feedback voltage value is smaller than a voltage lower limit valueof the voltage threshold range.
 20. The control method of claim 19,wherein the unit time value is equal to the time between any twovalleys.